The present invention pertains to acrylic polymerization systems and to diacyl peroxide catalysts useful in such systems, and particularly to such systems and catalysts used in dental restorations.
Acrylic polymerization systems are well-known in the art. They are described in, for example, Bowen U.S. Pat. No. 3,066,112; Lee, et al., U.S. Pat. No. 3,539,533; and Gander and Potts, U.S. Pat. No. 3,835,090, which are incorporated by reference into the present application. These systems are particularly useful as dental materials and generally comprise at least one liquid polymerizable acrylate or methacrylate monomer, a diacyl peroxide polymerization catalyst or initiator, and an amine accelerator for producing free radicals upon reaction with said catalyst thereby polymerizing the monomer. The systems may further comprise inorganic fillers and stabilizers.
These dental materials are generally prepared as two separate compositions for storage and distribution prior to use, one composition containing the catalyst and no activator and the other containing activator and no catalyst. These two compositions may be both liquid, both pastes, one liquid and one paste, one liquid and one solid, or the like. Conveniently, the two compositions may be both liquids (unfilled) or pastes (composite); the systems of the invention are thus exemplified.
In one type of system, each composition contains a liquid polymerizable monomer (also referred to as the binder) and an inorganic filler (if desired) as the major components. The first composition (denominated the "catalyst composition") contains in addition to these common ingredients an appropriate amount of a diacyl peroxide polymerization catalyst, typically benzoyl peroxide. The second composition (denominated the "activator composition" or "universal composition") contains in addition to these common ingredients an appropriate amount of an amine accelerator for the catalyst, typically N,N-bis-(2-hyydroxyethyl)-4-methylaniline in prior art activator compositions. Each composition usually further contains appropriate amounts of stabilizers to prevent polymerization or discoloration of the separate compositions prior to mixing or discoloration of the polymerized composition.
Polymerizable dental materials are used for a variety of applications. They may be used in direct filling of teeth to replace tooth structure which has decayed. They may be used to fill fissures in healthy teeth to prevent future decay. They may be used as glazing compounds over composite restorations to provide a smooth, glossy surface. They may be used as cement in the attachment of dental bridgework and the like to a tooth stump. They are well-known in the dental art as being superior dental materials, especially for anterior teeth.
A dental material should possess thermal stability during storage prior to use. Thus, while the material should gel in a relatively short time once the two compositions are mixed, they should be able to be stored as two separate compositions for an extended period of time, preferably for at least six months at room temperature. To achieve this stability, stabilizers are often added to prior art compositions to prevent premature polymerizations.
Prior art catalysts are generally diacyl peroxides as disclosed in Reinhardt U.S. Pat. No. 3,256,254 and Bafford U.S. Pat. No. 3,580,955, which latter describes methods of making some and provides a comprehensive list thereof. Because of their use of these prior art catalysts, prior art dental materials generally have poor thermal stability. That is, the individual catalyst compositions are not indefinitely stable, and may begin to polymerize even before they are mixed with an activator composition or may discolor, thereby providing material of unacceptable quality. Stabilizers are generally added to the catalyst compositions to increase its stability, but only a small amount of these can be added without adversely affecting the reactivity. The useful lifetime of the catalyst compositions of the prior art is approximately 6 to 10 months at 70.degree. F.; at higher temperatures (e.g., 100.degree. F.), it is even shorter. That the useful lifetime of prior art catalyst compositions is so short is a serious disadvantage thereof. This lact of thermal stability has led users of these restorative materials to keep them cooled to prolong their useful lifetime. That such cooling is necessary to prolong their useful lifetime is a further disadvantage of prior art acrylic restoration materials.
One who attempts to remedy this deficiency in thermal stability by altering the composition of the acrylic dental material must be careful, however not to impair the other desirable properties thereof. Thus, for example, it would be possible to extend the useful lifetime of catalyst compositions by adding greater amounts of stabilizer, but such would decrease the reactivity of the compositions when mixed with accelerator and consequently would increase the time between mixing and hardening of the polymer (the gel and set times), which increase is undesirable.
Although prior art polymerizable dental restorative materials have enjoyed success, especially in the area of highly filled restorations, they have had poor thermal stability.
It is therefore an object of the present invention to provide acrylic polymerization systems having improved thermal stability without impairing any of the presently-existing desirable qualities of the system.
It is a still further object to provide novel diacyl peroxides which may be used as catalysts in said acrylic polymerization systems, which catalysts provide improved thermal stability while at the same time not impairing other desirable properties of the system.
These and other objects of the present invention will become apparent as the detailed description thereof proceeds